1. Field of the Invention
The invention generally relates to a method of operating a microlithographic apparatus, for example a projection exposure apparatus or a mask inspection apparatus. The invention particularly relates to EUV apparatus in which a real image of an array of reflective optical elements is produced on a mask to be imaged. Subject of the invention is also an apparatus which is suitable to perform such a method.
2. Description of Related Art
Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to light of a certain wavelength. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be imaged onto the photoresist. After exposure the photoresist is developed to produce an image that corresponds to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system that illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example. The apparatus further comprises a mask stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) that images the portion with the illumination field on the mask onto the photoresist, and a wafer alignment stage for aligning the wafer coated with the photoresist.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, with high integration densities more components can be produced on a single wafer, which increases the throughput of the apparatus.
Various approaches have been pursued in the past to achieve this aim. One approach is to improve the illumination of the mask. Ideally, the illumination system of a projection exposure apparatus illuminates each point of the field illuminated on the mask with projection light having a well defined angular light distribution and energy. The term angular light distribution describes how the light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular light distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be imaged onto the photoresist. For example, relatively large sized features may require a different angular light distribution than small sized features. The most commonly used angular light distributions are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular light distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
In the past the desired angular light distribution has often been produced by a diffractive optical element, a zoom objective and a pair of axicon elements. The diffractive optical element determines the basic angular light distribution, which can then be modified with the help of the zoom objective and the axicon elements. A drawback of this approach is that the flexibility to modify the angular light distribution is restricted. For example, changing from an annular illumination setting to a dipole illumination setting involves an exchange of the diffractive optical element.
It has therefore been proposed to use a mirror array to produce the desired angular light distribution. Such illumination systems are described in EP 1 262 836 A1, US 2006/0087636 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2, for example. In these illumination systems the mirror array illuminates an optical raster element that illuminates on the mask an illumination field having the desired geometry and irradiance distribution.
In the illumination system disclosed in US 2010/0157269 A1 the optical raster element is dispensed with. The mirror array is thus directly imaged on the mask. For that reason the mirror array has the same overall geometry as the field that is illuminated on the mask. At a given time, a point on the mask can be illuminated only from a single direction, which is determined by the tilt angle of the mirror that illuminates that point at that time. Nevertheless almost any arbitrary angular light distribution may be attained, because the point can be illuminated successively from different directions while it moves during a scan operating through the illumination field. The desired angular light distribution is therefore not produced simultaneously as in the other prior art illumination systems, but only after scan integration. An optical raster element, which is arranged between the light source and the mirror array, ensures that all mirrors of the array are illuminated by the light source in exactly the same way. This simplifies the control of the mirror array. Uniform illumination conditions on the mirrors also ensure that lateral shifts of a light beam illuminating the mirror array has no impact on the angular light distribution and light energy at mask level.
Another approach to improve the resolution of the apparatus is to reduce the wavelength of the projection light. Until recently the most sophisticated projection exposure apparatus used projection light having a wavelength of 193 nm, which is in the vacuum ultraviolet (VUV) spectral range. Meanwhile also projection exposure apparatus are available which use projection light having a wavelength of only 13.5 nm. This wavelength is in the extreme ultraviolet (EUV) spectral range, and therefore such apparatus are often simply referred to as EUV apparatus. Since there are no optical materials available which are sufficiently transparent for EUV projection light, such apparatus are of the catoptric type, i.e. they contain only mirrors.
The approach to let the mirror array illuminate the mask directly, i.e. without an intermediate optical raster element that destroys the imaging relationship, has significant advantages also for EUV apparatus, because it reduces the number of reflective surfaces that are required in the illumination system and thus helps to improve the throughput of the apparatus.